The spleen focus-forming virus (SFFV) and Friend mink cell focus-inducing virus (Fr-MCF) both induce eythroleukemia in susceptible strains of mice. Studies are being carried out to determine how these highly related viruses specifically interfere with erythropoiesis. Nucleotide sequencing of the SFFV envelope gene and comparison with the envelope genes of various ecotropic and MCF viruses has revealed a large deletion and two insertions that may account for some of the unusual properties of its product. Certain strains of mice are resistant to erythroleukemia induced by Fr-MCF virus. Since these mice endogenously express a novel MCF-virus-related glycoprotein, we have proposed that it confers resistance by preventing the replication and spread of MCF viruses. Genetic mapping studies are in progress to determine the number and location of the gene(s) conferring this referring this resistance. Erythroleukemia cell lines derived from mice infected with either Friend MuLV or SFFV were found to express high levels of a transformation-related protein, p53, and monoclonal antibody to p53 was found to co-precipitate a 70K protein in these cells. Studies are currently being carried out with these cells to determine if increased levels of any onc genes can be detected and to characterize the 70K co-precipitating protein.